Long-term Debt - Schedule of Junior Subordinated Debentures (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	$ 258.2	$ 257.8
Argo Group
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	$ 28.4	$ 28.4
Trust preferred debentures | Argo Group | PXRE Capital Statutory Trust II
Debt Instrument [Line Items]
Interest rate	4.26%	4.32%
Junior subordinated debentures, Amount	$ 18.0	$ 18.0
Trust preferred debentures | Argo Group | PXRE Capital Trust VI
Debt Instrument [Line Items]
Interest rate	4.12%	4.15%
Junior subordinated debentures, Amount	$ 10.3	$ 10.3
Trust preferred debentures | Argo Group US
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	$ 172.7	$ 172.7
Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust I
Debt Instrument [Line Items]
Interest rate	4.26%	4.32%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust III
Debt Instrument [Line Items]
Interest rate	4.22%	4.34%
Junior subordinated debentures, Amount	$ 12.3	$ 12.3
Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust IV
Debt Instrument [Line Items]
Interest rate	4.00%	4.07%
Junior subordinated debentures, Amount	$ 13.4	$ 13.4
Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust V
Debt Instrument [Line Items]
Interest rate	4.02%	4.05%
Junior subordinated debentures, Amount	$ 12.4	$ 12.4
Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust VI
Debt Instrument [Line Items]
Interest rate	4.02%	4.03%
Junior subordinated debentures, Amount	$ 13.4	$ 13.4
Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust VII
Debt Instrument [Line Items]
Interest rate	3.80%	3.82%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust VIII
Debt Instrument [Line Items]
Interest rate	3.76%	3.79%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust IX
Debt Instrument [Line Items]
Interest rate	3.80%	3.82%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust X
Debt Instrument [Line Items]
Interest rate	3.60%	3.62%
Junior subordinated debentures, Amount	$ 30.9	$ 30.9
London Interbank Offered Rate (LIBOR) | Trust preferred debentures | Argo Group | PXRE Capital Statutory Trust II
Debt Instrument [Line Items]
Interest rate basis spread	4.10%	4.10%
London Interbank Offered Rate (LIBOR) | Trust preferred debentures | Argo Group | PXRE Capital Trust VI
Debt Instrument [Line Items]
Interest rate basis spread	3.90%	3.90%
London Interbank Offered Rate (LIBOR) | Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust I
Debt Instrument [Line Items]
Interest rate basis spread	4.10%	4.10%
London Interbank Offered Rate (LIBOR) | Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust III
Debt Instrument [Line Items]
Interest rate basis spread	4.10%	4.10%
London Interbank Offered Rate (LIBOR) | Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust IV
Debt Instrument [Line Items]
Interest rate basis spread	3.85%	3.85%
London Interbank Offered Rate (LIBOR) | Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust V
Debt Instrument [Line Items]
Interest rate basis spread	3.85%	3.85%
London Interbank Offered Rate (LIBOR) | Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust VI
Debt Instrument [Line Items]
Interest rate basis spread	3.80%	3.80%
London Interbank Offered Rate (LIBOR) | Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust VII
Debt Instrument [Line Items]
Interest rate basis spread	3.60%	3.60%
London Interbank Offered Rate (LIBOR) | Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust VIII
Debt Instrument [Line Items]
Interest rate basis spread	3.55%	3.55%
London Interbank Offered Rate (LIBOR) | Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust IX
Debt Instrument [Line Items]
Interest rate basis spread	3.60%	3.60%
London Interbank Offered Rate (LIBOR) | Trust preferred debentures | Argo Group US | Argonaut Group Statutory Trust X
Debt Instrument [Line Items]
Interest rate basis spread	3.40%	3.40%